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Corrosion Protection
Presenter: Dr. Zalilah Sharer
© 2016 Gas Technology Centre
Universiti Teknologi Malaysia
27 February 2016
Cathodic Protection – Electrical Measurement and Monitoring
Part 3
Potential Measurement Equipment
1. Reference Electrodes • Cu/CuSO4
• Ag/AgCl
• Zinc
Potential Measurement Equipment
2. Voltmeter (multimeter) • high impedance ( 1 – 100 MΩ)
• BS 7361 specified minimum 10 MΩ
Potential Measurement Equipment
3. Recorders And Data Logging Equipment - when need continuous monitoring for eg DC traction effects, telluric effect or other earth currents - can incorporate with computer
How to measure potential?
1. Making electrical contact with the structures ---> test points 2. Placing of reference electrode ---> iR drop As close as possible to structures to be measured; but not in-
contact. Potential is shown by where the reference electrode is placed; NOT where the contact to structure is made !!
3. The use of coupon 4. Permanent reference electrode
How to measure potential?
BEWARE WHEN USING DIVER
ESPECIALLY FOR ICCP
ELECTRICAL SHOCK !!
How to measure direct current in CP circuit
Generally, insert ammeter in series in the circuit Ammeter , but with some errors due to added resistance
But, ammeter is o.k. for sacrificial anode system, but better with shunt (eg. 50mV drop scale)
Measure CP Current - shunt
How to measure direct current in CP circuit
For ICCP
current measurement needed at ……
T/R output and anodes
higher current, direct ammeter in series
not suitable
can used shunt + voltmeter (multimeter)
50A 75mV shunt = 0.0015 ohm ---> 1.5mV=1A
How to measure direct current in CP circuit
When more than 10A ---> can use “clamp-on meter” less than 10A, clamp-on meter has poor sensitivity thus less accurate
Special CP Surveys
1. Close Interval Potential Survey (CIPS)
provides overall protection level
normal test post locations are too general
record & experience shown failures due to
corrosion at away from test posts
can incorporate instantaneous off method
during survey
Close Interval Potential Survey (CIPS)
Close Interval Potential Survey (CIPS) for Cathodic Protection
Close Interval Potential Survey (CIPS) for Cathodic Protection
Close Interval Potential Survey (CIPS) for Cathodic Protection
Special CP Surveys
2. Pearson Survey to locate defects in buried pipeline coatings by a.c. signal between pipeline and remote earth 3. Current Attenuation Surveys to locate defects in buried pipeline coatings by monitor actual current flow in the line
Special CP Surveys
4. Direct Current Voltage Gradient (DCVG) to locate buried pipeline coating damage by detecting iR drop at the defect areas more accurate and fast
Special CP Surveys
Direct Current Voltage Gradient (DCVG)
Direct Current Voltage Gradient Survey (DCVG) for Coating Defect Detection
Direct Current Voltage Gradient Survey (DCVG) for Coating Defect Detection
Coating defect with Inadequate CP
Coating defect with Inadequate CP
Coating defect ready for repair
Coating defects detected by DCVG
Pipeline Current Mapper
Basic Theory: EM 1
Basic Theory: EM 3
Basic PCM Locating
Current Measurements
Current Measurements
When a fault is encountered the current quickly drops. A fault will result from coating damage, contacts with other services, etc. The loss of PCM+ current will be virtually proportional to the amount of CP current being used at the fault. NOTE: Linear current loss may naturally occur along the pipeline due to the
age of the pipe and coating conditions.
Data Interpretation
Other Measurement
Soil resistivity Meggar earth meter 4-pin Wenner Method Insulation checker checking insulation at IF, MIJ GPS Global positioning cordinates (useful for cross country CIPS & DCVG) Pipe Locators
How to measure the soil electrolyte resistivity
Most popular method ----> Wenner 4-pin method Driving 4 steel pins into earth in a straight line, equal spaced Pin spacing = depth
Soil resistivity = average resistivitiy at that particular depth
Four-pin Wenner Method
How to measure the soil electrolyte resistivity
On resistance meter…..
C1, C2 = current spikes
P1, P2 = potential spikes
Result on meter = resistance (ohm)
Resistivity ……..
(ohm-cm) = 2 π a R 100
a = pin spacing in meter R= resistance
(from meter)
Wenner Four Pin Method
Wenner Four Pin Method
Wenner Four Pin Method
Wenner Four Pin Method
Wenner Four Pin Method
Wenner Four Pin Method
Soil Resistivity Measurement
Soil Resistivity Measurement
How to measure the soil electrolyte resistivity
PRECAUTIONS : 1. Soil box sample ---> preserve normal moisture
content.
2. Measurement closely parallel to a bare underground pipeline ---> not accurate (usually very low values)
3. Ensure proper pin contact 4. Electrical shock
How to measure the soil electrolyte resistivity
Soil Box ---> soil samples
Collect soil sample to fill up the soil box, then measure resistivity by resistance meter. Use same arrangement, i.e. C1,C2,P1,P2
Soil Resistivity Measurements
Important step in CP Current requirement differ from one soil
resistivity to another No precise method to determine the degree of
corrosivity caused by soil, only give rough guide to the corrosivity of soil
Most methods used is Four Pin Wenner’s Method
Four-pin Direct Current Method
Significance of Soil Resistivity
Soil resistivity gives indication of the corrosivity of soil. Some approximation are given below;
Soil resistivity (ohm.cm) Corrosion
0-1000 Severe corrosion
1000-3000 Severe corrosion
3000-10000 Moderate corrosion
Over 10000 Very mild corrosion
Average resistivities of some soils
Environment Resistivity (ohm.cm)
Brackish river water 1
Sea water 25
Town supply water 1000-1200
Alluvial soil 1000-2000
Clay 1000-5000
Gravel 10000-25000
Sand 25000-50000
Mid Point Contact Surveys
Tow Fish Trailing Wire Survey
Close Electrode Trailing Wire Survey
Two Electrode Survey (PipeScan)
Three Electrode Survey (common)
Meter – Care Taking (Precautions)
Do not mesure values beyond range of meter
For current & voltage measurement, always use highest range first
Avoid dropping meter
Pack carefully for travel
Store properly
Question?